MAD2L2 is a component of the mitotic spindle assembly checkpoint that prevents the onset of anaphase until all chromosomes are properly aligned at the metaphase plate. MAD2L2 is a homolog of MAD2L1. The predicted 211-amino acid MAD2B protein shares 24% and 26% sequence identity with yeast Mad2 and human MAD2L1, respectively, in the conserved regions. RT-PCR analysis revealed that both human MAD2 homologs were expressed at similar high levels in a panel of cell lines. Further binding analyses determined that the interaction of MAD2L2 with ADAM9 is mediated through a proline-rich SH3-ligand domain of ADAM9. Northern blot analysis detected 1.35-kb MAD2L3 transcripts in all tissues tested, with highest expression in testis.